In a power conversion system such as a variable speed, constant frequency (VSCF) power generating system, a brushless, three-phase synchronous generator operates in a generating mode to convert variable speed motive power supplied by a prime mover into variable frequency AC power. The variable frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant frequency AC power, which is then supplied over a load bus to one or more loads.
As is known, a generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
A further way of operating a brushless, synchronous generator in a starting mode is disclosed in Dhyanchand, et al., U.S. patent application Ser. No. 07/421,640, filed Oct. 16, 1989 entitled "Brushless Generator Having Prime Mover Start Capability" and assigned to the assignee of the instant application, the disclosure of which is hereby incorporated by reference. The generator includes a rotor having a series of damper bars which are electrically connected to form a squirrel cage winding. During operation in the starting mode, power is applied to armature windings of the generator, in turn causing currents to flow in the damper bars and thereby causing the generator to operate as an induction motor. Such a starting system does not require position feedback for the inverter control, and hence the system is relatively simple in design.
Compoly, et al., U.S. Pat. No. 3,775,662 discloses a 24-step inverter having four inverter bridges which are coupled to four sets of primary windings of a summing transformer. The windings of these sets of primary windings are connected in a wye configuration and the windings of the remaining two sets are connected in a delta configuration. The transformer includes a set of secondary windings in which summed voltages are developed. The inverter bridges are operated at particular phase displacements with respect to one another to produce a 24-step, three-phase output.
Dhyanchand, et al., U.S. patent application Ser. No. 07/421,830, filed Oct. 16, 1989, entitled "Power Conversion System With Stepped Waveform DC/AC Converter Having Prime Mover Start Capability" discloses a power conversion system which is operable in generating and starting modes. The converter includes a rectifier bridge which is coupled to the output of a brushless, synchronous generator driven by a prime mover and first through fourth inverter bridges which are coupled to the output of the rectifier. First through fourth primary windings of a summing transformer are coupled to the outputs of the first through fourth inverter bridges, respectively. The windings of each of the first and third sets of primary windings are connected in a wye configuration whereas the windings of each of the second and fourth sets are connected in a delta configuration. The first through fourth inverters are operated to produce 24-step, constant frequency AC power in a secondary winding in the summing transformer. During operation in a starting mode, an external AC power source is coupled to the secondary winding of the summing transformer, the input of the rectifier bridge is coupled to the wye-connected primary windings and the outputs of the first and third inverters are coupled to armature windings of the generator. The first and third inverters are controlled to convert the DC power produced by the rectifier into AC power for causing the generator to operate as a motor and thereby develop the required motive starting power.
Other power conversion systems operable in generating and starting modes are disclosed in Cook, U.S. Pat. No. 4,786,852 (assigned to the assignee of the present application), Shilling, U.S. Pat. No. 4,743,777 and Messenger, U.S. Pat. No. 3,908,161.